High-resolution palaeoenvironmental reconstruction of the mid-Barremian Munk Marl Bed, Danish North Sea based on calcareous nannofossils

The mid-Barremian (Early Cretaceous) was characterized by an episode of widespread deposition of laminated black shales in the Boreal Realm (Hauptblätterton in northwest Germany, Munk Marl in the North Sea), interpreted as a regional oceanic anoxic event (OAE). In order to better constrain the palaeoenvironmental setting of this event, well-preserved calcareous nannofossil assemblages of two North Sea cores have been studied across the Munk Marl. Due to their abundance in pelagic sediments and sensitivity to environmental change in sea-surface waters, these phytoplanktonic algae constitute an excellent proxy for changes in temperature, fertility and distance to the land. Our results show an increase in the relative abundance of Watznaueria barnesiae together with a decrease in abundance of fertility index taxa across the Munk Marl. We also observe a threefold compartmentation of the Munk Marl. The lower and upper part of the Munk Marl are characterized by low abundances of Micrantholithus spp. and Rhagodiscus asper (coastal and warm water proxies) whereas the central part shows a high abundance in Nannoconids (oligotrophic indicators). Our results thus suggest that the mid-Barremian event was associated with warmer temperatures and lowered productivity in sea-surface waters, triggered by a warm and arid climate and reduced terrigenous run-off